Your answer can be found on PhotographerMom’s post #1952 on the wait list thread that is linked below, there is nothing you can do to get off the waitlist as there are numerous variables that go into waitlist results (school’s overall yield, yield within applicant’s particular pool, # of applicants on waitlist, etc.) I suggest moving forward with plan-B and wait late march/early April to see if there will be any waitlist movement (which is almost always very minimal at Andover and Exeter).
Waitlist thread: http://talk.collegeconfidential.com/prep-school-admissions/1300302-the-wait-list-p131.html